1. Field of the Invention
The field of the invention is that of optical gyros, notably used in the field of inertial navigation in the context of embedded aeronautical applications. More precisely, the field of the invention is that of optical passive resonator gyros.
2. Description of the Related Art
Optical gyros are based on the principle of the measurement of the Sagnac effect. Under the effect of a rotation, the Sagnac effect induces a difference in traversal time between two electromagnetic signals propagating in opposite directions along a ring-shaped path. This difference in traversal time, proportional to the angular velocity of the device, can be measured either as a difference in phase in the context of an interferometer assembly or as a difference in proper frequency between the two counter-rotating modes of a ring-shaped cavity. In the first case, it is necessary to use an optical fibre to maximize the length of the interferometer and therefore the sensitivity of the device. The term interferometric fibre optic gyro, known by the acronym I-FOG, is then used. In the second case, the difference between the proper frequencies of the modes of the cavity can be measured in two ways. The first consists in using an active cavity, i.e. containing an amplifying medium, and in measuring the frequency difference between the counter-rotating modes emitted by the cavity. The term RLG, an acronym of Ring Laser Gyro, is then used. The second way consists in using a passive resonator cavity and in probing the proper frequencies of the counter-rotating modes using a laser. The term passive resonator gyro is then used.
The passive resonator gyro has a certain number of advantages compared to its rivals. Compared to the RLG, it notably circumvents the need to use a gaseous amplifying medium and the high-voltage electrode system that is usually associated with it. Compared to the I-FOG, it exhibits the advantage of a much shorter optical path which allows it to be less sensitive to the environment as well as more compact. Finally, it only involves standard components. Thus, the use of super-luminescent source is in particular avoided.
However, although these three types of gyros, I-FOG, RLG and passive resonator gyro, have all been demonstrated experimentally, to date, only the first two have resulted in industrial applications. The problem of backlighting, which creates coupling between counter-rotating modes and degrades the performance of the system, has been a brake on the development of the passive resonator gyro. Currently no completely satisfactory technical solution has been found to this problem. The noise resulting from the interference between the useful signal and the back-reflected signal can be partly filtered by a phase modulation technique. However, the latter in no way solves the problem of the coupling between counter-rotating modes, which creates a non-linearity in the frequency response and leads to a “blind spot” as on conventional ring laser gyros. Additionally, the problem of the stability of the scale factor is usually not dealt with in this type of gyro.